The present disclosure relates generally to the field of well drilling.
Generally, when drilling a well, the pore pressure gradient and the fracture pressure gradient increase with the true vertical depth (TVD) of the well. Typically for each drilling interval, a mud density (mud weight or MW) is used that is greater than the pore pressure gradient, but less than the fracture pressure gradient.
As the well is deepened, the mud weight is increased to maintain a safe margin above the pore pressure gradient. If the mud weight falls below the pore pressure gradient, a number of well control issues may arise, for example taking a kick. If the mud weight exceeds the fracture gradient, the formation may be fractured resulting in lost circulation and its associated problems.
To prevent the above situation from occurring, conventional practice typically involves running and cementing a steel casing string in the well. The casing and cement serve to block the pathway for the mud pressure to be applied to the earth above the depth of the casing shoe. This allows the mud weight to be increased so that the next drilling interval can be drilled. This process is generally repeated using decreasing bit and casing sizes until the well reaches the planned depth. Because well costs are primarily driven by the required rig time to construct the well, these processes may increase the cost of drilling the well. Furthermore, with the conventional steel casing tapered-hole-drilling process, the final hole size that is achieved may not be useable, or optimal, and the casing and cement operations substantially increase well costs.
Because of the time and costs associated with running casing strings, it is desirable to drill as long of an open hole as possible. A multi gradient drilling system enhances this capability.